Feeding UHF into a little VHF amp?

Discussion in 'General Technical Questions and Answers' started by KM6LYW, Aug 9, 2018.

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  1. KM6LYW

    KM6LYW Ham Member QRZ Page

    I just started using a little 30watt VHF HT amplifier (Radio Shack HTA 20) with my little 5watt yaesu. It works perfectly! However, I have UHF memories in my scan loop, and I'm worried I'll accidentally transmit on UHF through the VHF amplifier at some point.

    Any ideas on what will happen if I do? smoke? nothing? Destroy amp? HT? both?

    curiously,
    -craig
    KM6LYW/AG
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I strongly doubt you'd damage the amp.

    But the input match would be really lousy on UHF and there's a possibility you may damage the HT (depends a lot on which HT, how long the coax is between the handheld and the amp, and other variables).
     
  3. KM6LYW

    KM6LYW Ham Member QRZ Page

    Okay, so an inadvertent transmit might not cause smoke, thanks. HT is yaesu FT1XDR, with five feet of rg58 to amp. I don't honestly know if the Yaesu HT's will throttle back if SWR is off the scale.
     
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I don't know, either.

    One sure-fire way to prevent a problem (but it costs $$) would be to use two diplexers: A VHF/UHF diplexer at the amplifier input, and another one at the output.

    The "common" port of diplexer #1 goes to the HT. The "VHF" port of that diplexer goes to the amplifier input.

    The "common" port of diplexer #2 goes to the antenna. The "VHF" port of that diplexer goes to the amplifier output.

    The "UHF" port of diplexer #1 connects directly to the "UHF" port of diplexer #2.

    Now, the amp will only see an input (drive) signal on VHF and will never see it on UHF -- at all.

    The two diplexers completely bypass the amp on UHF.

    The diplexers are often called "duplexers." In this application, same thing -- doesn't matter which term is used.

    [​IMG]
     
  5. AG6QR

    AG6QR Subscriber QRZ Page

    Careful. You may have created a feedback loop, from the output of the amp, through the output diplexer's VHF to UHF side, through the coax to the other diplexer, and finally through the input diplexer's UHF to VHF side into the amp input.

    The UHF and VHF sides of the diplexers should be well isolated for most purposes, but when using them like this, you really need superb isolation. If the amp's gain is greater than the total isolation along the path from amp output to amp input, bad things will happen.

    I don't know what the typical isolation of those diplexers is, but I'd be sure and check it before connecting things that way.
     
    KM6LYW likes this.
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I've measured every one I ever used (mostly from Comet and Diamond, but they're all very similar) and 146 to 440 MHz port isolation >60 dB.

    The brick amps usually have gain in the 10-16 dB range, depending if they're one stage or two cascaded stages.

    I've done exactly what I described many times.
     
    KM6LYW likes this.
  7. WG8Z

    WG8Z Ham Member QRZ Page

    I done this more than once , works fine.
     
    KM6LYW likes this.
  8. KM6LYW

    KM6LYW Ham Member QRZ Page

    this is too good to be true. passive circuit? what kind of loss do I have to pay? i'd probably just buy one and hook a little uhf whip up to it, or dummy load. all the vhf would go to my hi gain antenna thru the amp... right?

    thanks,

    craig
     
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    It's passive and losses are so low you probably can't measure them. I've measured my "Diamond" (brand - Japanese) units at 0.1 dB through loss.

    But it takes two of them to do this, not one.
     
    KM6LYW likes this.
  10. AG6QR

    AG6QR Subscriber QRZ Page

    OK, that's better than I had feared. I won't argue with success.
     

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